It is a challenge to assess the contents of container in a non-intrusive way. The desire to do so is often fuelled by the threats posed for example by radioactive or explosive contents.
For such purposes, passive and active assessment systems exist. Passive systems can be in the form of radiation portal monitors (RPMs). Such systems are composed of radiation detectors that detect radiation emitted by radioactive substances within the bulk in question. The prime drawbacks of conventional technologies used in RPMs are the following:                1. Plastic Scintillator technology (PVT), as disclosed in the U.S. Pat. No. 5,679,956: Has very poor energy resolution, preventing isotope identification. This leads to frequent innocent alarms due to naturally occurring radioactive material (NORM) or technically enhanced NORM (TENORM) material.        2. There are attempts to surmount these insufficiencies by using scintillating crystals (such as Nal(Tl)). The price of crystals prevents production of large devices, limiting their scalability.        3. Commonly used scintillation materials (such as the examples above) are not capable of distinguishing neutron from gamma ray signals.        
Efforts have been made to perform such a task by using passive radiation detectors (see for example WO-A1-2005/009886 or U.S. Pat. No. 5,679,956 or U.S. Pat. No. 6,768,421 or US-A1-2005/0029460). The shortcomings of commonly employed solutions are various: Detectors with sufficient energy resolution to give clues to the source identity are too costly to use for a detector with a large detection volume. Further, to detect neutron-emitting substances, separate neutron detectors need to be installed, as most radiation detectors are not capable of discerning between neutron and gamma radiation (see the US-A1-2005/0029460).
A problem with passive monitoring is nuisance alarms caused by naturally occurring radioactive materials (NORM) and technically enhanced naturally occurring radioactive materials (TENORM). To avoid such incidents, it is desirable to have a detector that can measure the spectrum of the detected radiation in order to identify the source isotope. A further way to reduce nuisance alarms is the use of a detector capable of discerning strong point sources from conglomerations of weak sources in a large partial volume. A powerful distinction criterion between benign and harmful radioactive sources is the uniformity of emission. While benign sources often are large volumes of low activity material, dangerous sources often are much more point like.
Active systems consist of a radiation source of some and a detector of some form. The bulk in question is illuminated by the radiation source referred to as the interrogating radiation. The radiation of the radiation source is commonly in the form of neutrons and/or gamma rays. The detector can measure the effects of the materials within the bulk in question on the interrogating radiation. Such effects may be attenuation, scattering, or neutron resonance. The detector may also measure radiation emitted upon stimulation by the interrogating beam, due to physical processes such as stimulated fission or photo fission.
The invention is a response to the demand for new technologies for the assessment of materials in a bulk volume. In particular, for homeland security purposes, systems capable of quantifying the threat posed by containers, trucks, trains, or other freight forms are seeked. Explosives or radioactive substances for use in dirty bombs or nuclear weapons may pose these threats.
The challenge can be stated as the following: Conceive a detector whose scalability to large dimensions is feasible from a technical and from a financial point of view, which is capable of detecting and discerning neutrons from other forms of radiation, as well as giving the most precise possible information regarding the energy of the incident radiation. The detector ideally provides means for identifying point sources.
On the other hand, radiation detectors using noble gas have been used for radiation detection from small scales up to very large scales. For example, the ICARUS collaboration has deployed neutrino detectors with hundreds of tons of detector volume. In most noble gas based detectors, the ionization charge brought forth by energy radiation interactions is measured.
It has already been suggested that different particles, i.e. alpha particles, electrons, and fission fragments, lead to different scintillation pulse shapes in liquid argon and liquid xenon (see for example Hitachi et al. Phys. Rev. B, 27(9), p. 5279-5285 (1983)). This effect is assumed to be brought forth by the fact that different particles interact differently with the target material, transferring their energy either to target nuclei or target electrons, or to a combination of the two. The effect is also assumed to depend on the density of energy deposition.
Recently, it has been proposed (Boulay et al., Direct WIMP Detection Using Scintillation Time Discrimination in Liquid Argon, arXiv: astro-ph/0411358v1 (15 Nov. 2004)) to use this fact to detect dark matter in the form of WIMPs (Weakly Interacting Massive Particles). The proposed detector is based solely on the detection of liquid neon or argon scintillation light to discern between WIMPs and the internal background caused mainly by beta radioactivity proceeding from detector components, in particular radioactive impurities in the noble gas. The method of discrimination relies on the different scintillation light pulse shape emitted by beta as opposed to assumed WIMPS interactions.
Passive monitoring procedures are commonly used to detect illicit radioactive sources in containers. For this purpose, plastic scintillators are often employed, detecting the gamma ray count rate. Efforts have been made to construct and deploy detectors relying on scintillating crystals in order to measure the radiation spectrum and identify the source isotope.
Active interrogation techniques have been proposed, where the working principle includes a radiation source and a detector. Material assessment is performed, relying on physical effects such as induced fission, photo fission, nuclear fluorescence, and beam attenuation.
Liquid noble gas ionization drift chambers have been proposed for active interrogation with cosmic muons.
Imaging techniques using gamma rays (Compton imaging) or neutrons are described in scientific publications. For neutrons, a good description of this technique is given in 2005 IEEE Nuclear Science Symposium Conference Record, “Demonstration of a directional Fast Neutron Detector” by P. E. Vanier.